The present invention relates to an image forming apparatus including an electrophotographic copying machine or a laser-beam printer. The present invention is particularly concerned with the improvement of a developing unit for developing an electrostatic latent image by supplying toner to the electrostatic latent image.
As a developing unit carrying out an electrophotographic process to be installed in an image forming apparatus, a two-component developing unit and a single-component developing unit are known. The two-component developing unit uses toner and carrier to develop an image, whereas the single-component developing unit uses toner alone.
In the two-component developing unit, development is performed by allowing toner particles to adhere to carrier particles, transferring the carrier particles with the toner to the outer periphery of a development sleeve to form a developer layer thereon, bringing the developer layer into contact with the surface of a photosensitive drum, removing the toner particles from the carrier particles by the Coulomb force of the electrostatic latent image previously formed on the photosensitive drum, thereby attaching the toner particles to the electrostatic latent image.
In the single-component developing unit, development is performed by forming a thin film of toner particles alone on the outer periphery of a development sleeve, and arranging a photosensitive drum to face the development sleeve at a predetermined interval, thereby supplying toner particles selectively to an electrostatic latent image previously formed on the photosensitive drum to attach the toner to the electrostatic latent image.
Recently, Jpn. Pat. Appln. KOKOKU Publication Nos. 7-40156 and 7-43554 have proposed a xe2x80x9cquasi two-component developing unitxe2x80x9d using a small amount of magnetic carrier. The quasi two-component developing unit basically belongs to the class of single-component developing units. In the quasi two-component developing unit, development is performed by attaching a starting material containing toner and a small amount of magnetic carrier to a development sleeve before a developing process is initiated and then supplying the toner alone to the development sleeve. Alternatively, a developer is known that comprises a magnetic toner and a small amount of magnetic carrier, the developer being supplied in the vicinity of a developing roller to supply only the magnetic toner to the development sleeve.
In the two-component developing unit and the quasi two-component developing unit, a doctor blade is arranged near the development sleeve so as to face the peripheral surface thereof. Since the doctor blade cuts and removes excessive developer from the development sleeve, a developer layer (containing carrier and toner) can be formed with a predetermined thickness on the development sleeve.
In such a two-component developing unit, the developer cut and removed is properly returned to a toner hopper, where the carrier and the toner are agitated and so mixed up and again supplied to the developing roller (the development sleeve).
However, in the quasi two-component developing unit, the developer cut and removed is returned sequentially through a toner hopper, a toner table (stepped portion of a housing) and a developing roller. This means that the magnetic carrier is returned together with the toner to the toner hopper when the developer is returned to the toner hopper. As a result, shortage of magnetic carrier locally occurs in the vicinity of the developing roller. Since the amount of magnetic carrier is originally low in the quasi two-component developing unit, shortage of magnetic carrier occurs frequently. If the supply of magnetic carrier is insufficient, the concentration of toner present on the developing roller varies, producing an image of uneven density and a faintness, and fogging.
A conventional quasi two-component developing unit generally employs a vertical transfer system. Therefore, toner is supplied to the developing roller by simply being raised from a toner-stirring vessel. It is therefore difficult to maintain uniform toner concentration.
An object of the present invention is to provide an image forming apparatus capable of maintaining a stable image quality for a long time without producing an image of uneven density, a faintness or fogging. Particularly, the present invention is directed to a developing unit for maintaining the toner supplied to a developing roller at a uniform concentration.
The image forming apparatus of the present invention comprises
an image read-out section for reading out an image;
a photosensitive drum for forming an electrostatic latent image corresponding to the image based on image data supplied from the image read-out section;
a vertical transfer passage for transferring a transfer material upward to the photosensitive drum; and
a developing unit arranged below the photosensitive drum for depositing the toner on the electrostatic latent image formed on the photosensitive drum with the assistance of a small amount of magnetic carrier.
The developing unit comprises
a housing;
a toner hopper, which is a part of the housing, for containing a toner;
a toner supply source for supplying the toner to the toner hopper;
a magnetic roller positioned above the toner hopper for imparting a magnetic field to the magnetic carrier;
a development sleeve, covering the outer periphery of the magnetic roller, holding the magnetic carrier, allowing the magnetic carrier to adsorb the toner, rotating in the opposite direction to the rotation direction of the magnetic roller, thereby allowing the toner to transfer and adhere onto the electrostatic latent image formed on the photosensitive drum;
a doctor blade arranged a slight distance from the development sleeve for cutting and removing excessively supplied toner from the development sleeve;
an agitator arranged within the toner hopper for agitating and mixing the toner contained in the toner hopper;
a rotation arm arranged in the agitator and rotated about a rotation center lower than a rotation center of the development sleeve;
a spoon having a substantially flat spoon surface (bowl) at a tip of the rotation arm for scooping up the toner from the toner hopper when the spoon surface (bowl of the spoon) comes close to the development sleeve and transferring the toner;
a toner shelf, which is a part of the housing, arranged immediately under the development sleeve for holding the toner transferred by the spoon and for receiving the toner and the magnetic carrier cut and removed by the doctor blade;
a developer returning member, arranged between the doctor blade and the toner hopper, for preventing the toner and magnetic carrier cut and removed by the doctor blade from transferring to the toner hopper, guiding and returning the toner and magnetic carrier to the toner shelf; and
a weir, arranged between the toner shelf and the toner hopper, having a top portion which is equal to or higher than the spoon surface (bowl of the spoon) in height when the rotation arm is horizontal and which defines a space between the top portion and the developer returning member for passing the toner to be transferred by the spoon, and guiding and returning the toner and magnetic carrier cut and removed when the developer returning member returns the toner to the development sleeve.
The top portion of the weir is desirably higher by up to 2.5 mm than the spoon surface (the bowl of the spoon) when the rotation arm is horizontal. This is because, as shown in FIG. 5, if the difference xcex94h in height between the top portion of the weir and the spoon surface (the bowl of the spoon) is lower than 0 (xcex94h less than 0), the concentration of the toner on the development sleeve becomes uneven. In particular, if the difference xcex94h in height is less than xe2x88x921.2 mm (xcex94h less than xe2x88x921.2 mm), the concentration of toner on the development sleeve becomes significantly uneven, causing development defects. This phenomenon occurs in the region leftward of line E in FIG. 5.
On the other hand, if the difference xcex94h in height exceeds 2.5 mm (xcex94h greater than 2.5 mm), the amount of toner supplied to the development sleeve decreases, producing a faint image. In particular, if the difference xcex94h in height exceeds 3 mm (xcex94h greater than 3 mm), letters become too faint to read. This phenomenon occurs in the region rightward of line F in FIG. 5.
Furthermore, when the toner concentration on the development sleeve increases, letters become faint. In particular, when the toner has run out, the toner concentration exceeds 18 wt %, with the result that printing cannot be done properly because of faint streaking. This phenomenon occurs in the region above line G in FIG. 5.
According to the present invention, it is possible to suppress the supply of toner to the development sleeve so that the carrier/toner on the development sleeve can be charged more efficiently. Thus, the toner particles on the development sleeve are uniformly charged, with the result that uniform development can be achieved. Hence, it is possible to obtain stable images having no unevenness and faint streaking.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.